Loft / mesh issue

Hi all! 

I am trying to make a rectangular cutout with rounded edges. I used to do this with a booleandifference operation in Rhino, but since I have to do this many times, for different shapes, I have attempted to write a Grasshopper script to automate the process. 

In the script, I use a loft to connect two surfaces. As you can see in the image below, something strange happens in the corners of this loft... When I bake the shape, the problem is less apparent, but when I export to an STL mesh (which I need for 3D printing) irregularities are created in the corners. Does anyone know what is causing this problem? Is it the loft or is there something wrong with the curves / surfaces I am using? 

I also tried adjusting my mesh settings, but this didn't work either...

Cutout.gh